This invention relates to a hydraulic control system for the four-wheel drive transfer mechanism connected to an automatic transmission for changing the four-wheel drive transfer mechanism between two ranges, i.e. a direct-coupling range (high-speed range) and a reduced-speed drive range (low-speed range) as well as between two-wheel drive and four-wheel drive ranges.
Generally, a transfer mechanism for four-wheel driving is changeable between two-wheel drive direct coupling range (referred to as "H.sub.2 " hereinafter), four-wheel drive direct coupling range (referred to as "H.sub.4 " hereinafter) and reduced speed four-wheel drive range (referred to as "L.sub.4 " hereinafter). In a four-wheel drive transfer mechanism directly connected to an automatic transmission, wet multiple disk clutches and wet multiple disk brakes, which are controlled hydraulically, are employed as frictional engagement means which is controlled to engage and disengage to attain the H.sub.2, H.sub.4 or L.sub.4. The hydraulic control system for controlling the changing operation is operated by the control oil pressure of the automatic transmission.
Changing between the direct coupling range and the reduced speed range in the four-wheel drive transfer mechanism is carried out by changing supply of the hydraulic fluid between the respective hydraulic mechanisms of the direct-coupling frictional engagement device and the reduced speed drive frictional engagement device, by means of an inhibitor valve (high-low selector valve). The timing of changing between the direct coupling range (high-speed range) and the reduced speed range (low-speed range), particularly the timing of shifting down from the high-speed range to the low-speed range, is a significant factor affecting generation of shocks in changing speed and the running performance of the vehicle.
For example, in changing the transfer mechanism from H.sub.4 to L.sub.4 while a four-wheel drive vehicle is running up a steep slope in H.sub.4 to cope with a requirement for a greater driving force, the transfer mechanism is set temporarily in the neutral mode during the changing operation when the reduced-speed drive frictional engagement device is engaged after the direct-coupling frictional engagement device has been disengaged and the vehicle thereby may unexpectedly run down the slope, which is dangerous. Accordingly, an overlapping changing operation is more preferable.
However, in changing the transfer mechanism from H.sub.4 to L.sub.4 while the vehicle is running along a level road at a low speed, the overlapping changing operation will cause an increased changing shock, which is undesirable. In this case, therefore, it is preferable to engage the reduced-speed drive frictional engagement device after the direct-coupling frictional engagement device has been disengaged.
However, the aforementioned inhibitor valve (high-low selector valve) alone can not set a change speed timing corresponding to the running state as described hereinbefore.